you probably have a leak on your shower valve.

Hey. Unfortunately if there is a wet spot which does not dry the same colour as all the other grouting areas that definitely means there is a leak somewhere.

Hi,
Sorry to know about this issue.
Leakage could happen due to gaps in grouts, broken tiles and/or damaged silicone. If you can, you can thoroughly inspect the area and figure out the problem and fix it. You can buy grout from any shop (B&Q, Homebase, Wickes...etc) or Silicone and apply then. Or a good DIYer or a tiler can help.

愿一切都好！

Hi, it always possible that you have a small leak if that area remains constantly wet. However, another reason may simply be that the holes around the shower valve (where pipework penetrates the tiles) has not been sealed correctly with silicone sealant allowing water ingress. You have not mentioned whether or not the walls within the shower area have been tanked, so I am assuming that is not the case. Sadly no tile or grout (save for epoxy resin grout) are waterproof they are only water resistant, thus the importance of always tanking walls & floors which constantly come into contact with water, such as showers in order to protect the substrates from water penetration and therefore damage.

Hope this helps.

I'm not a plumber but it sounds to me you have a leak on the pipework behind the tiles

这听起来像是有小漏连接ions at the back of the shower mixer unit. Two pipes (hot and cold 15mm) will be connected at the back of the thermostatic unit. It may be that whoever fitted it may not have tightened the nuts fully and/or not used PTFE tape. How old is the shower installation? It's a good idea to have a plumber isolate the water supply and remove the unit to identify the leak. Water will be getting down any cavity and if there's a small leak coming through a break in the grouting, it means that the leak is either in front of the backer board or on the junction of the board and tiles. From the sound of it, it is likely that water will be getting down any cavity behind the backer board and eventually reaching the floor.
Best to get it sorted sooner than later since, while the backer board won't swell, any damp timbers will eventually swell and potentially cause further damage.