Bath Sealant

Thats why I always put slicone down first in a FAT bead, and tile UP to the bath.

I'd hope your floor isn't flexing! What about the bath itself? Plastic ones, unless *very* expensive, tend to flap around like a sheet in the breeze. In particular they can sag significantly when full (or water + person). A small amount of movement will strain a 2mm sealant bead.

Silicone doesn't always bond well, especially if there's any hint of damp on the surfaces (which is almost inevitable with repair jobs). Nowadays I use Stixall from Toolstation (which I think is a knock-off of Sticks Like Sh*t from Evo, though I haven't tried the latter). This does stick in the wet, and can be smoothed down with a finger wetted in dilute washing-up liquid, as for silicone.

UP ???

If its not too late you can get sealing strips that fit behind the tiles that should last a lot longer.

Use Evostick Nail and Seal. It's a modified silicone co-polymer that has better adhesion and superior mechanical properties to silicone. It's easy to clean and resists abarasives and can be sanded if necessary (*not* recommended when used on a bath).

The only negative with it is that it's available in one colour only, white.

Teleseal?

Mitch brought next idea :

The stick on strips do work very well. Its a matter of making sure the area is scrupulously clean and apply pressure all the way along them to get them to stick firmly - a roller does this best. The only problem I have had is with slight shrinkage back away from a cut corner.

With a plastic bath, I would suggest filling it up with water before starting, so as to make it deform to its maximum.

Attach the bath permanently to the wall using something like solvent based gripfill. Keep the bath full of water as it cures so it fixes in the "full" position.

I find that 12mm plastic quadrant (buy it from plastic facia board suppliers) is cheap and very effective. Just bed it down on good quality silicon.