Hello,
Sorry to hear this.
I never heard about such a behavior. But if it occurs for you, it must be a bug of 6.14 version.
Regarding the latest FDM: I've tried the magnet you provided - it started to download immediately with a good speed.
The only thing I can suspect is that some trackers are blocked by your ISP. FDM 6.14 probably had a base of recent peers saved in its DB. But, the format was changed later so newer FDM can't understand it. That's why the latest FDM can't download such torrent for you. Maybe you need to just wait a bit longer for the DHT (for example) to acquire peers.

I disabled both the "Check for updates automatically" and "Install updates automatically" under the Preferences menu, yet the FDM still kept auto-updating, how to fix this ?

The reason I prefer the old version 6.14 is because some of the torrent and magnet downloads can NOT be started with Ver.6.19, it shows "There are no connections" forever, yet these downloads work perfectly fine with Ver.6.14.

For instance:

magnet:?xt=urn:btih:1EC9A535B29ADCEADA95659F6A8A4B808A1D3B6E&dn=The.Flash.2023.1080p.WEB-DL.H.264-RiGHTNOW%5BTGx%5D&tr=udp%3A%2F%2Ftracker.coppersurfer.tk%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.bittor.pw%3A1337%2Fannounce&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337&tr=udp%3A%2F%2Fbt.xxx-tracker.com%3A2710%2Fannounce&tr=udp%3A%2F%2Fpublic.popcorn-tracker.org%3A6969%2Fannounce&tr=udp%3A%2F%2Feddie4.nl%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.torrent.eu.org%3A451%2Fannounce&tr=udp%3A%2F%2Fp4p.arenabg.com%3A1337%2Fannounce&tr=udp%3A%2F%2Ftracker.tiny-vps.com%3A6969%2Fannounce&tr=udp%3A%2F%2Fopen.stealth.si%3A80%2Fannounce