Caulk problem in tub

Did you have the tub filled when you caulked? The weight of the water pushes the tub downward and if you caulk at that time, the stuff just compresses as the tub is drained and the tub rises. If you caulked and then filled the tub, the caulk would likely tear and let go.

Acrylic caulk is good but Silicone II (100% silicone) is generally regarded as more flexible and longer lasting.

I think acrylic comes in more colors and has a flatter sheen and now comes mixed with antimicrobials. 100% Silicone has a glossier sheen and usually only comes in white and clear but you can find black and blue at automotive stores for gasket making but it has other fillers.

Since you need to recaulk anyway, try the silicone II and fill the tub as the other poster suggested.

When I did my tub (surface mounted) it had a rather large gap. I filled that first with expanding foam which I trimmed so that it was only inside then spread caulk on top of that to form a fillet. This seems to be holding up well.

Silicone caulk should be used. If the tub moves that much, I would suspect problems with the floor.