If recent pageid:nnnn is occurring in search expression, the possible pages and namespace locations are already known.
When Special:Search is used, that introduces some profile=default and other context which results in namespace limitation, especially consider ns=0 only.
If pageid:nnnn is occurring in search expression, all namespace limitations shall be dropped, since they prevent results when nnnn is located outside main space.
An easy workaround is to use the all: prefix which will unlock all the namespaces from the query string, e.g. all:pageid:XYZ.
I'm not sure that pageid should have an impact on the namespace as one of its usecase is to intersect the page ids with other filters (the namespaces being part of these).
The major purpose is to have as first step some results from incategory: and any keyword search.
Any namespace limitation is pointless if a set of pageids is provided.
Namespaces shall be ignored in the same way as by all: if pageid: is present.
We had a discussion with the members of the search team and we think it's preferable for this keyword to not interfere with the set of namespaces being filtered.
Reasons are that it allows for more usecase and an easy workaround using all: prefix exists.
Usecase that would not be possible: