Well of course, there is no guarantee the leak is actually getting by the edge of the shower tray. The tile joint in the corner or even a grout line may be cracked. Is the tray loose?? It is possible water could be getting into the shower cubicle framework and leaking past the tray there. You haven't sealed from the inside of the shower screen have you???
If general sealing doesn't work, you need to remove the screen entirely, examine the tray for movement, check all tiles are fixed securely, re-seal the tray then replace the screen and re-seal. A chore I know, but it's the only way and the longer it leaks the less likely you will be able to cure it.
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