Yeah it's like
@Brian G. said. Once you go into the past and change stuff, those changes only affect branch-offs of the past timeline. The changes have no effect on the "present time MCU"...
So Steve could screw up anything in the past timeline and it'll just create alternate branch offs, those have no effect on the present. He could have died and never came back, and it wouldn't change anything in the present.
That's why it's pretty clear that he had to use the machine to return to the present with the new shield for Falcon. Anybody in the past timelines or branch-offs has to leap into the present timeline to be present.
Same applies to 2012 Loki, 2012 Loki uses the Tesseract to escape, but all his adventures after that (the Disney+ premise) will only affect branch-off timelines. Only if he makes a "leap" into the "present time MCU" can he have any impact on the present.