For certain words or word-combinations the spellcheck complains about a correct word-with-postfix ignoring its postfix and suggests to replace it with word-with-postfix which then duplicates the postifx and makes it wrong.
Examples:
-- en_GB --
'Inc' in 'Inc.' suggesting 'Inc.'
-- de_DE_new --
'pseudo' in 'pseudo-wissenschaftlich' suggesting 'pseudo-' (okay, the dictionary has 'pseudowissenschaftlich' but is not suggesting that)
'pseudo' in 'pseudo-alternativ' suggesting 'pseudo-' (the dictionary has not 'pseudoalternativ')
Now imagine what this does to 'Prof. Dr.'