I've found a strange occurrence, and I don't know which part of it isn't supposed to happen.

While in my party, there's a monk character (he has the highest virtue and knows the most saints), others also have knowledge of saints so that he's not the only one spending DF. I've found that when we're donating money to a church, all characters' DF value go back to 99, no matter how low it was, EXCEPT the monk's.
Now, I don't know whether it's a bug that donating gives such an easy way to regain Divine Favor or that my monk (thus, the one who'd mostly need this boost) is excluded from it, but it's strange nonetheless. Anyone got some insights about it?

When I offer the donation with the main char as a leader, he, alongside with the alchemist guy, gets his DF up to 99, but the monk doesn't. It's not possible to split the party, if that's what you're asking.
Praying is the more traditional method of regaining DF, which is available to everyone, but it only gives a few DF points a day.

What about making the monk leader and donating with him/her?

Same result as with any other character: he doesn't get any DF replenishment. I've even tried donating twice, but even that had no effect (first time it filled the alchemist's DF bar, then it did nothing).

Going to confession is a free way to quickly regain DF but it takes time and only the party leader gets DF restored. (You can switch party leader ofc)

Yes, thanks, I know about that, and also that asking the monks to pray for us also gives some points of DF. I knew about such methods - what bugs me is why only my monk is left out of the insta-filling effect of donations.

Is he the primary saint user? Maybe the more you use saints the harder it is to refill Divine Favor. This is just a wild guess not backed up by any solid evidence or anything.

He is, but I'll probably just use the others from now on when possible (i.e. only use my monk for saints whose Virtue requirement is so high the other members can't invoke the the saint). Also, I don't think there's such a discrimination against "primary saint users".
And it seems now that the monk is definitely bugged: now he can't even replenish his DF by attending masses or going to confession! They work (thus, replenish 2-3 points) for other characters, but not him.

See, it's difficult to say whether this a bug in the original game or if this is GOG-specific. Sounds to me like it might be something with the game as there have been issues reported with overflow errors when something goes over 99, like storing things in an inn.

Only thing I can suggest is sending your save file to support and see what they come up with.

I may just have to do that... But I doubt they can do anything with it. I guess it's a non GOG-specific bug, but it's worth a shot anyway.

Never know until you try.

Too bad disassembler programs just don't give out the source code in the proper language so others can see where mistakes were made. If I could find a program that did that I would be more than happy to teach myself how to code so I could work things out like this.

It is possible to learn how to recode a binary well enough to see where mistakes were made, even dumb stuff the compiler did which the source code would not reveal.

definitely a bug
my guess is he's the last character in the group. try donations with a Hanse or a Schultz member in the no 5 slot. number 4 will work then but not no.5 but who cares. Marching order doesn't help and the only way to avoid this is by going to the inn letting go of 2 people and bringing them back in a different order.
just don't forget to remove all items before you let them go, they come back with nothing.

also find your min max donations for instance a party with all characters above 30 virtue that goes to church and donates receive a bump of 3 virtue if they donate enough gold so if you have 140 florins exchange all but 33 florins with the Fugger or Medici.then donate 3x exchange and go back next day if you have gold virtue is pretty easy to build
so if your going for virtue you want more florins if you just need divine favor bring much less this may change with religion but never found relig of much use except when praying for df