Shower controls get tight when hot

[maui19]

Our newly renovated shower has a weird issue. The shower control gets very tight when it gets hot. It is ok (and I mean just ok) when at room temp, but after a few minutes of a hot shower, the control gets very hard to move, and untimately turning off the shower requires two hands. I can't get traction on this with either the plumber or the contractor, but I haven't really pressed them on it (yet). The valve is a Moen M-Core mixing valve (MOE-U362CIS). The diverter works fine even when hot. The valve and trim are all brand new. Any ideas what might be causing this? Thx!