The rendering problem in interaction with Convert Forms and SP Page Builder can be traced to a CSS conflict in the stylesheet of the template. However, it has now been resolved via a CSS adjustment in SP Page Builder. If it is similar to others here is the solution that you have to enter in the Joomla module CSS in the SP Page Builder:
.convertforms .cf-input {
font-size: 1em !important;
color: var(--input-color) !important;
background-color: var(--input-background-color) !important;
padding: var(--input-padding) !important;
border: solid 1px var(--input-border-color) !important;
border-radius: var(--input-border-radius) !important;
text-align: var(--input-text-align) !important;
}
Problem could thus be solved and the post can be closed ;)