I'm quite sure this issue is not due to timing issues on my end.
After some additional testing, this is the specific way to reproduce the behavior:
When a site requests a TOTP to log in, activate Enpass Mini
Using the arrow keys, highlight the desired account from the list of Enpass items
Use the right arrow key to open the account details
Use the down arrow key to highlight the TOTP field
Take note of the time remaining, wait until a fresh OTP is generated, and take mental note of the number
Press enter to copy the TOTP and close Enpass Mini
Press ctrl+v to paste into the TOTP field for the website
The pasted TOTP will differ completely from the one displayed (which you took a mental note of)
This bug cannot be reproduced by manually clicking the 'copy' button on the TOTP nor by using the 'copy TOTP' keyboard shortcut (ctrl+shift+t) - both of these produce the correct/expected result.