I may be erring a bit too far on the "close enough" side having been aiming for no-cost fixes at the local bike co-op lately, but I'd ride it and see if it happens again. While that particular design may have been a press fit, it isn't unusual to see EBBs (eccentric bottom brackets) designed with a slip fit between the spindle and bearings, then a set screw/shaft collar to prevent lateral movement. As a new to you bike, it's possible it took a good shove on the side of the crank, was mis-installed previously, etc.
I'm under the impression you had to give it a few taps to get it back in to place. If it slips freely back and forth, or if you come back from a ride and see that it's happened again, then I'd take a bit more action. You could probably set it with a single drop of loctite after cleaning the spindle, but take care not to let any get out to the seal. It's not the end of the world, but loctite between the inner race and the seal doesn't do the seal any favors. Replacing the spindle or adding collars (if there is space) would be the next course of action, but it I don't see this as a critical item the way a scored pedal spindle, shifting rear derailer alignment, etc. are, IE it isn't something likely to result in injury, additional costly repairs, or walking home if it does recur.