1m on reels and under 1k on tiktok with the same file usually isn't luck, and you can check it in about two minutes.
open analytics on one of the sub-1k tiktoks and look at two things. traffic sources: if for you page is under roughly 70%, you never cleared the first test batch, it got shown to a small cold pool and stopped there. and full watch rate: under 20% on a short clip means the opening lost them, distribution was never the problem.
the part that trips people up is that reels leans on your account history and your existing followers. tiktok mostly doesn't. every upload gets judged close to from scratch, in front of strangers with no reason to stay past second two. so a video your ig audience opens instantly can flatline on tiktok without anything being wrong with the account.
also worth saying since you're posting the identical file: if it goes to instagram first and tiktok second, you're handing tiktok an export it can tell has been compressed and posted elsewhere. i'd try one week of tiktok first, same content, different opening frame. that tells you more than any hashtag change.