During WW1 the spotting aircraft operated near the apex altitude of the shells and routinely reported seeing them as they reached their peak and slowed down, being buffeted by passing artillery and some instances of aircraft being hit, some by the shells from the battery they were supporting.
There were several reports by pilots of seeing naval 15" rounds in the air while supporting the D-Day landings and Normandy campaign, WCdr Johnnie Johnson was one of them. A naval spotter flying a Seafire flew thru the slipstream of a 15" round and was spun out of control, recovering just before impact. As in WW1, there were instances of spotters being hit by artillery.
There is this photo of a
CARIBOU hit by a friendly 155mm shell while on approach to land at Ha Thanh Special Forces Camp.
Cheers,
RichB