I just came across the following, which may relate to your issue, even though you've created a custom template. This issue appears to be making changes to the template, however, not simply opening a document.

If Word is being used in a networked environment, the Normal.dot file could be common to all users on the network. If this is the case, then changes cannot be saved to Normal.dot when other people are accessing it. For instance, if User 1 and User 2are using Word, both are actually accessing the same Normal.dot file. When User 1 exits the program, his changes cannot be written to Normal.dot until User 2also exits the program.