Reported by Paiy on Discord.

The test level shows the bug; Faller checks fail when they miss a 1px high water hitbox. This bug is not present in SuperLemmix 2.6.1; upon investigation, it's been introduced by the recent water physics updates in 2.7 - specifically, I refactored all of the water objects to be checked as a group in order to simplify the code. This is because Invincible lems are allowed to swim in "unswimmable" water objects (i.e. Blasticine, Vinewater, Lava) - so, I wanted it all to be handled in a single method.

However, due to the way lemming position checks are carried out, all water objects need to be handled separately, otherwise checks such as the one identified here can tend to fail. I thought I'd caught them all in the refactoring, but obviously not. So, it's all back as it was previously, with each water object being checked and handled separately from the others.

This has been fixed in Commit 0dfeb6fd3. A hotfix will be released shortly.