and yes..there can be a problem mixing leaded and unleaded enamels. You have just been lucky. Speaking to the experts at Thompson enamel, where unleaded enamel is manufactured, they advised me not to use unleaded on the back as counter enamel if I am going to use leaded on the front.
From Thompson Enamel’s WORKBOOK regarding using lead-free and
lead-bearing enamel, “Certainly, lead bearing can be applied to one
side of a form and lead free on the other side, if the expansions are
close.” In addition, “…if lead free is applied underneath, lead
bearing enamel can be fired on top (after firing the lead free)
within the same cloison.”
Regarding circular-type cracks, as I recall, Darty states that this
type of crack is due primarily to COE (expansion) incompatibilities
(metal vs. enamel -OR- enamel vs. enamel).
BTW, Thompson’s WORKBOOK has a chart of all of the COE’s, Fusion
Flow, and Dilatometric temperatures for each of their enamels. Very
helpful. Ellis, Helwig and Carpenter wrote the WORKBOOK and at $9.98
it’s jam-packed with enameling (not well edited, but
every sentence has value). Highly recommended for any enamellist.