Also make sure you have tightened up the adjustable feet under the bath. You say it happens when showering etc. That may because more weight is placed at one end of the bath pulling away from all your sealant, closing again when weight is off.
The bath also settles in time, adjustable feet sinking into chip board flooring or distribution batten that you may have used. Worth popping the panel off again and making sure all is tight!
These plastic baths should be installed with a timber baton fixed along the back edge to support bath , if not the bath will flex & split the sealant , or better still cut the back edge into the wall & make good , i realise it's a bit late now.
When you originally did the job, did you fill the bath with water during application and drying of the sealant? If not, there's a good chance that it's shifted since then, hence the leaks.
I haven't really researched the cladding you used, but presumably it's completely watertight, and what you're seeing definitely isn't water getting behind the cladding and running down the back of the bath?
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