1. if the fen position does not have pieces down a file, than the file collapses (it does not happen in boards in pgn

for example use 2r1kb1r/pp4pp/2n1p3/1B2N3/5KbB/P4P2/1P5P/2R4R b k - 0 1

and you will see it breaking along the d file

2. The square_size does not work if you select the pop_up navigation board

Besides those 2 the plugin is really great (and beatiful , which is very important).

to be exact , the second one is not a bug . probably its a decision.