I've been noticing that sometimes when a t or d follows a final -n it becomes n. For instance, one friend (from Nanjing) pronounced 今天 Jīntiān as Jīntnian, and another (from Beijing) pronounced 运动 yùndòng as yùnnong. Searching around it seems others have noticed this too.
When in casual speech does this change occur? Since the neutral tone is prevalent in native Mainland Mandarin speakers how are homophones understood? Like 今天 vs 今年