Merged cells aren't editable when the cells below are having different sizes and the left one is smaller than the right one. If you then click in the midlle of the merged cells you can't edit the cell. You need to go to the beginning or the end of the cell to be able to edit the text in the cell. It is only editable till the end of the first column, not after the end of the cell.
Created attachment 76943 [details] example with table and merged cells
Confirmed. See attached example. Reproducible since version 3.3 on Linux and Windows
submitted a patch for this bug here -> https://gerrit.libreoffice.org/#/c/3273/ People willing to test can git pull. Give your feedback/review if you are tested the patch.
karth committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=bd1e5791951c0e8eae6e79f1789ba1115cdead53 Resolves fdo#62673 Table cells non-editable on cell merge The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
Fix verified on a build from master.