This is a feature of
biblatex's punctuation tracker. A
. can have two meanings for
biblatex. It is either a dot to signify abbreviations or a full stop (period) to indicate the end of a sentence.
. is treated as a full stop by default; this behaviour can be reversed by issuing
\isdot, then a preceding
. is treated as an abbreviation dot.
prenote formatting (
biblatex.def) we find
. in a pre-note are seen as abbreviation dots, which makes sense since a prenote often contains a short abbreviation such as "Cf." or just a short word such as "see", but rarely an entire sentence.
There are two solutions here.
One is a temporary solution in which you tell
biblatex to capitalise the next word anyway
The second solution will globally switch the behaviour in prenotes to recognise
. as full stops.