From the few examples that I have been able to find within either Workbooks or MTIP, the single 5s (few and far between) seem to coincide either with held base notes, with
tied notes or note values that extend beyond a beat e.g. base descending in crotchets whilst instrument above holds a
dotted crotchet.
Do be patient though, there are many on this forum whose command of Figured Bass is far greater than mine; let's hope someone comes along soon!