If people experience this behaviour in combination with ContentBlocks, it may be caused by ContentBlocks removing the standard content field (which avoids issues with Redactor). That behavior is configurable and can be disabled to avoid conflicts with TinyMCE. See this FAQ for info.
TinyMCE (4.3.4) is based on TinyMCE 3 and the MODX version of it is no longer actively maintained. Our recommendation is to use TinyMCE Rich Text Editor (based on TinyMCE4) or Redactor as RTEs for MODX at this time.