Last week I flew to Sandown, IOW, on the last day before UK Lockdown No. 2.
The circuit was very busy due to airfield operational issues so I had to fly three circuits before landing.
I didn't do "touch and go" - I just continued to fly at circuit height.
Each time I overflew the airfield I received a warning about an aircraft at 35000ft above, overtaking at 330 Kt.
I think this warning occurred three times.
I didn't see anything on the moving map but wasn't really looking for long either due a fairly busy circuit.
I think SD was detecting high level CAT in the airway above me, so likely a glitch in the SD traffic filtering algorithm.
I have never detected anything similar at any other time and just use the standard default for a height filter in the basic SD setup.