Patch before that no problems,
I used the same file for the first playthrough and I ended the game at about level 50-ish.
Now when loading any game existing before Blood and Wine I gain a lot of xp points and then about 30 levels gain after destroying a monster nest.
It also happens on NG+saves where I'm still in WO.

The saves made in first playthrough are fine, so I could continue from there,

but I really wanted a shot at the g***t in the beginning and I guess I was very eager to explore when I first got the DLC so that I didn't save untill about when you get to meet this certain lovely royal for the first time, so yeah...

Anyone?

edit I never used any mods, tried to verify through GOG, and loaded some old beginning of the game files to check if it affected all. It did ):. All the saves created before release of blood and wine have it.

Pretty obvious a bug as before patch all okay and using no mods.

Ah ok..
Do you think I should report it somewhere or is mentioning it here enough?

Mentioning it here will get you - perhaps - help from a member that had (and solved) exactly your problem, while sending in a support ticket at CDPR will get the attention of the folks that might actually change things (might take some time).

Hi. No solution, I'm sorry. I'm just dropping in to say that I am feeling your pain. I have a similar issue:

https://www.gog.com/forum/the_witcher_3_wild_hunt/level_70_after_reload_from_level_54

Too early to say whether I can reproduce this or not -- what would be the first thing they asked at CDPR: They can't fix bugs they can't see, can they.

I've been getting the same result every time I load a pre-blood and wine save.
So it's pretty consistant on my end.
I'll try to send in report ticket and I'll keep you informed.
Thanks everyone!

Hi! For me it seems to be related to potion of clearance
I was gonna start playing blood and wine today and started from a Hearts of stone save from january (patch 1.12) and encountered the same problem. I was lvl 37 but had like "59500/2000" xp so after I killed a mob I got to lvl 66.
However, this was a save that I had just used potion of clearance in. I instead loaded my save just before I used the potion and I had normal amounts of xp.