Many times. Growing up, mom was always present in the exam room, and every visit I can remember ended with a shot in the bottom. So my mom at the minimum saw my butt at every doctor visit all the way to age 18. Only one time it was my dad instead of my mom.
At 6, when I had my tonsils out there was very little privacy and lots of rectal thermometers. I don't remember a whole lot, as a small child I remembered the ice cream the most.
Somewhere around age 9 an entire family walked in on me while I was laying nude on the exam table waiting for a shot. My mom would always make me "get ready" for my shot by taking off my underwear and making me lay face down on the table until the nurse arrived. After this incident, I talked my mom into letting me stay in my briefs until the nurse arrived.
At age 12, the doctor walked in while the nurse was taking my rectal temperature. Although it was a doctor in this case, the door was wide open for a number of seconds, and there was no way to hide, with my briefs at my knees and the nurse holding the thermometer in my bottom.
At age 29 I was in the E.R. and marched back to get x-rays in a hospital gown that did not cover my butt at all. The E.R. was so packed that there were beds in the corridors, with visitors hanging around, so a number of people probably saw my butt.
The E.R. is probably the most non-private medical care facility, I have never been in the E.R. and not seen partial nudity either from the curtains not being closed all the way (and they never are) or from gowns not being closed all the way (they usually aren't either).