Did I make a mistake while transferring BTC to my cold wallet?

Noob here.

Last week I transfered some fiat and converted it into BTC. I then made the smallest transaction possible (0,0008 BTC, with a 0,0004 BTC fee) toward my cold wallet (Tangem) and it went through perfectly. I then transferred the rest of my BTC toward my cold wallet, but a few hours later the transaction failed. Here is where it gets weird : – I received a few emails mentioning I will get an answer within 3 to 5 business days (so sometime during this past week), but I still haven’t receive anything back – the c.com support tells me that it will be transferred back into my bank account (which doesn’t accept BTC obviously, and they don’t even have my bank info) – there is an “adjustment (transfer)” that seems to be pending, no further details available, but it is approximately 0,001 BTC (100€) less than the initial transfer. Support tells me that it might be due to price fluctuations (but to transfer 0,1 BTC from the exchange to a cold wallet is still 0,1 BTC – fees right?) – support tells me not to worry (they are right, we are just talking about my life savings here) and that it should be resolved “soon” (no further details available, it could be hours or months, nobody knows) – when I asked “why was it cancelled” they told me “because it didn’t went through” (no duh)

Any idea what went wrong? What could I have done to prevent it? Is there a possibility my savings are all gone?

Thank you for taking the time to read through all of it.