The lack of variation within heavy fourth declension nominals gives rise to similar difficulties.
Equally problematic is the fact that fourth declension nominals occur with the exponents -t/-de/-sid, irrespective of syllable count, and that all second declension nominals in -ja, etc.
Approaching the same question from a more psychological angle, one is naturally led to ask why a speaker should fail to exploit the predictive value of the genitive or nominative singular of a fourth declension noun solely because the corresponding form would be an unreliable predictor in other noun classes.
The 'minimal' fourth declension paradigms would, correspondingly, come to represent the productive pattern for light disyllables.
OS 2006 identify fourth declension forms such as 'koide as being consistently Q3, in contrast to second declension forms such as 'puude, which are marked as being optionally Q3.