Which version of Pega are you using? I found a link where it is written -
RTE does not display in pop-up windows
The RTE field does not display when it is configured for pop-up window. The same RTE field displays when it is configured on other screens other than the pop-up window. This functionality worked as expected in Pega 7.4. Users reported this problem after upgrading to Pega 8.1.2. This regression was fixed in Pega 8.2.2.
From Pega 8.x onwards the hotfixes will be provided in extremely rare and exceptional situations (such as production down or security vulnerability) as per Pega's policy.
Going forward, fixes will be delivered through the patch release process only.
@NITESH AGARWAL I am not sure of any local-change as of now as this is Product Bug which has been fixed in 8.2.2. You can raise an INC and add all the reference articles. And also provide business justification for the requirement.