I initially thought this was just a block display glitch but in testing it for this issue it appears to be slightly more glitchy.

If you make a Modular Storage and put a Storage Module in it, the bar graph on the front will display as empty. The tooltip for the module will read 0/x00 stacks.

Place 1 item into the inventory, the bar graph now may still read as empty until the GUI is closed and opened and then closed again. The tooltip will read 1/x00 stacks, the GUI will show [Z] 1/x00.

Open the GUI, remove the module, replace the same module. The tooltip will now say 2/x00 stacks however the GUI will read correctly as [Z] 1/x00. Removing the module from the machine resets the tooltip to 1/x00 stacks. The jump in stack count increases as the used storage also increases e.g. 42->55.

Removing and replacing the module will cause the bar graph on the front to randomly display between empty and 1/8 full even if the module is empty. Removing the module always makes the graph reset to appear as empty.

Thank you for taking the time to create this issue, as we're no longer supporting versions below 1.12, I'm going to go ahead and close it but if this bug seems to continue over to newer versions, please do report it again so we can investigate the issue further.