No expert here, but it seems like your tension is off when sewing with the grey thread: the top thread is floating on top of the fabric and pulls the bottom thread up a little everywhere and it pulls the bottom thread up a lot when you sew points. It doesn’t seem to be as bad with the beige thread. The difference may be b abuse the threads are different and it could be the interaction between the top and bottom thread. Generally, you should reduce the upper tension when it is pulling the bottom tension up. The general rule is to avoid — if possible — adjusting the bottom tension, but if it pulls off the bobbin too easily, you may not have a choice.
Other things to try: different thread, usually with the same in the top and bottom to make that tension adjustment easier
Is your presser foot adjustable? Make sure it just barely touches the fabric, neither too high or low.
If you change your thread, you may need to change the needle size.
Rethread your machine and your bobbin. Sometimes there is a nonobvious error, even if you have sewn on the same machine for years.
Getting the right tension is a matter of trial and error and lots of practice sandwiches. I feel for you as I have definitely been there, done that. And then, for whatever reason, it just starts to work. Best of luck to you!