Product and version FileMaker Pro 15.0.1 & FileMaker Pro 15.0.2 (doesn't do this in FM14)
OS and version OSX 10.11.6
Hardware MacBook Pro (Retina, 15 inch, Late 2013)
Description A text block that has some text in it and is formatted to align to the bottom will grow by however many pixels you have given to the bottom padding every time you click into the text box, whether you edit the text or not
How to replicate add a text box to a field & enter some text; set the text box to have some padding on the bottom & set it to align to the bottom; click outside of the text box & note the height of the text block; click back into the text block to edit it & then click outside the text block. The height of the text block has grown by however many pixels you gave the padding.
Workaround (if any) whenever you edit the text, resize the height of the box after you've finished editing