Just had a new shower installed, tiles going down and overlapping the
flange of an acrylic shower base. Contractor "sealed" the tile/base
connection with grout, which soon cracked due to weight in shower and
flexibility of base. I then used grouted caulk, which now has also
cracked.

How can I seal this small gap in a way that will not crack? Or maybe I
should not worry about it (concrete backboard behind tiles).

TIA