Mh, let me check where the size hint should come from and why it might be missing in my windows test. That is indeed strange then.

For me this does work also when decrypting:

btw. this does not work on the decrypting side because kleopatra there apparently does not provide a size hint. So it goes:

Due to back porting another change the fix for 2.2 is now also needed in 2.4.

works with Gpg4win-4.1.1-beta295

I checked the upstream. For the reported issue, upstream version raises an error with REG_ERR_UNMATCHED_BRACKET.
That behavior is better (as we don't have particular reason to maintain different behavior from upstream version).
Also, I found another change from upstream for end of word check.

To replicate the problem it is best to use Windows. Should be solved with my commit. Note that the bug is specific to 2.4 dues to irts multi-card and app support. There was no problem on 2.2.

my Yubikey works, too, if I disable PIV. With enabled PIV:

Unfortunately I can't replicate that with my Yubikey on 2.4.1. Tried several variant and with and without keyboxd. My Yubikey has PIV disabled but I doubt that this is the problem.

Test with GnuPG 2.4.1-beta76 failed with "error getting current key info: invalid name":