This has been noted by several users in the OH 4.3 Discussion thread. What worked for me and others is to add a stateDescription metadata to the slider item with a step unit of 1.
N1k145
I also can confirm this. All sliders of dimmer items linked to an mqtt:dimmer channel are stuck at position 0 after upgrading to 4.3.
A workaround for me was setting a state description “step” of 1.