You might argue that there is a small conflict of interest arising from the citation, namely that your paper being published slightly boosts the citee’s bibliometric scores. However, if reviewers were excluded due to this, many of the most qualified reviewers would be among them to the extent that it may be impossible to find any remotely qualified reviewer in smaller, citation-heavy subfields. Moreover, whom you cite is obvious to the editor, so if they really want to take this into account, they can.
Of course, a connection between your work and another that results in a citation may as well as reflect a conflict of interest that is not obvious or comprised by what is obligatory to report (as per the venue’s rules), but that has nothing to do with the citation per se.
That being said, there can be tactical reasons not to suggest authors you cite as reviewers:
When an editor has to select reviewers, your citations are a primary source of inspiration.
(Some authors even carefully arrange their citations due to this.)
Thus, suggesting somebody you prominently cited is redundant and potentially wastes a suggested-reviewer slot which you could have used to suggest somebody who is an excellent match for your paper but whom you did not cite prominently or at all.
I think of the suggested-reviewer list as a list of reviewers who would be very suited to review the paper but are not an obvious choice.
Moreover, many editors make a point of choosing at least one reviewer you did not suggest.
Thus it can be beneficial to leave them some obvious choices to “discover” by themselves.