TASCAM gear does some things in quirky, unexpected ways, which can be frustrating to figure out sometimes.
It could be something that needs to be set or changed via the menu, or it could be some funky button/switch combo, or a bit of both.
I don't own one of these FOH mixers, so the only other advice I can offer is to download the pdf version of the owner manual and make a search on "solo", "monitor", etc.
The other is to try tracing the control room signal flow through the Block Diagram in the OM.