Kinda sounds like a bad switch, normally its a relatively easy fix with a new replacement 3pdt type switch, but (i’m assuming it the new modern LBM?) these are soldered to a mini circuit board with a soldered ribbon cable to the main circuit board, so the best thing though its more complicated is to replace the switch and ribbon cable with new wires from the circuit board to a new 3pdt, but you need to know where each wire connects to, can’t help much there but it can be figured out with patience and going thru the circuit.

or…

i had a similar type of problem with my newer LBM, where i had thought the switch quit, thus it only worked in bypass mode. i found that the ribbon cable is made of pretty cheap thin wire and if fiddled with enough (doesn’t take much) it can fail and stop making contact at certain wire/solder points, i had to resplice and resolder the ribbon cable where it connected to the 3pdt miniboard to get the effect to work again. this may be the case for you, that means the 3pdt is fine but the ribbon cable connections have gotten loose making intermittant or no contact…

…but if this is the old style LBM, i assume its using either a dpdt or spdt switch which has stopped working and thus its not true-bypass, i suggest replacing with a 3pdt and true bypass it.