Why is it so?

Your VoIP and IPTV VLAN settings are still LEFT untouched inside your BTU. All you have to do in future is just swap back your Thompson to have the service reenabled.

Better still if you can convince the TM technician can configure the BTU to strip the VLANs on one of the other 3 ports. Leave port 1 as it is.
Just enable port binding on either port 2-4 (whichever you want) and untick VLAN tagging. Leave LAN port 1 as it is.

I'm sure many of those who subscribed the RM118 package earlier won't be bothered about the the IPTV and calls. It's very high posibility the pay TV charges will be incurred extra.

So yeah, just leave port 1 as it is and make the BTU unstrip the VLAN on the other ports for the use of your 3rd party router.
Any service you want to continue using in future should you change your mind, just plug your Thompson into port 1. The TM technician will have to make another visit to your home anyway if there's any changes.