This is a disgrace and a bodge job in my opinion and will crack and loosen in no time
These plastic channels that have been used here are not for sub base use but are off the shelf and if they were intended to be used for such purpose they would be more substantial in their design
Also fashioned I humbly and with having years of construction experience suggest with pre moulded external side pertruding lugs which would ensure that the channel is anchored down by the surrounding bonding material
Looks like the project manager has given the job to his mate up the pub tbh
As a last ditch effort I would see if something like this could now be the solution
If/when it becomes loose again maybe suggest such a thing
A scroll through the links images show such use as yours
Hope it gets permanently sorted
