Yes, here we have it Update Values in App and BasicUI - #23 by ssalonen
The automatic conversion happening with Dimmer items seems to prefer on/off = 0/100 interpretation. By having transformation, even that dummy identity transformation, it goes correctly.
This is buried in the docs https://www.openhab.org/addons/bindings/modbus/#transform-on-read
The default is to convert non-zero numbers to ON/OPEN, and zero numbers to OFF/CLOSED, respectively. If the item linked to the data channel does not accept these states, the number is converted to best-effort-basis to the states accepted by the item.
With Dimmer item the ON/OFF unfortunately takes precedence over the numeric value