A well-known sakt occurs at man rāq in Sūrat al-Qiyāmah. The regular nūn-to-rā assimilation is not applied here; the voice pauses briefly and resumes at rāq.
Assimilating the n into r would miss the prescribed form. Taking a long breathing stop would also change the nature of sakt.
Practice: complete the n, pause momentarily without breathing, then pronounce r.