Am I correct that the problem was not immediate? That it seemed to appear after use to become more and more pronounced?
If so, I’ve had the exact same experience to discover two culprits:
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Loose assembly screws throughout the machine. If you don’t already have them get a pair of “jeweler” type hex drivers with a swivel head. Read through various replacement instructions at the Bambu A1 wiki to essentially disassemble the entire machine to access as many fasteners as reasonably possible. Tighten the fasteners in related groups and then repeat to ensure that they are all snug. This certainly includes every screw in the printing bed and its platform!
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The other problem is uneven Z (vertical) movement that results in the X-axis becoming progressively out of parallel with the printing bed. This [seems] to start with a firm collision between the print head and a partly printed object of sufficient force to either knock the part off the plate or cause the machine to detect the collision, stop printing, move the print head to a “safe” location and display a message an error message with the possibility to resume printing. Once this occurs the problem seems to increase with continued “normal” operation with the misalignment eventually become very noticeable to the eye. While bed leveling and mechanical geometry of the process are intended and designed to properly deal with this exact situation problems seem to creep up as the misalignment increases. To correct follow the instructions for Z-axis belt replacement. This is not a difficult job. Provided the belt is in good condition there is no need to replace.