I have the same problem, since version 6.3.0 (Win10 / x64) and contained until now 6.3.1.2, the output found was to use version 6.2.5.2, when the file is not mine, and my file started using geometrical figures or images instead of button.

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Tested on 19/09/2019
Em 6.3.2.1(x64) + Win10, problem remains.