Just for future reference: I have checked the literature. The cluster trait is actually monogenic, but recessive (i.e. the single fruit trait is dominant). So the phenotype will first reappear in F2 after crossing and additional candidates will appear in F3 (and later).
See e.g.:
Curr. Sci 118:1598 (2020) Inheritance of fruiting habit traits in chilli (Capsicum annuum L.)
Indian J. Genet. 49:219 (1989) Inheritance of clusterness and fruit orientation in chilli (Capsicum annuum L.)
Mysore J. Agric. Sci 52:293 (2018) Genetics of fruiting habit traits in chilli (Capsicum annuum L.)