My MacBook Pro M5 running MacOS Tahoe 26.3 beta fails to detect two identical ASUS ROG Swift OLED PG32UCDM monitors simultaneously. Only one display is recognized at a time.
One potential root cause might be that both monitors report identical binary EDID serial numbers (0x01010101), and the MacBook Pro M5 appears to use this value exclusively for display identity rather than combining it with other more detailed information (e.g., port, or alphanumeric serial number).
I've verified that the monitor EDID binary serial numbers are in fact identical -- however the alphanumerical serial numbers are not identical.
NOTE: This behavior is specific to the MacBook Pro M5 — when connecting both monitors via usb-c to a Mac Mini M4 Pro running the same MacOS Tahoe 26.3 beta, the monitors work fine. The OS detects both and assigns different names to them (PG32UCDM (1) and PG32UCDM (2)).
NOTE: I could be wrong about this root cause, I don't have a way to disprove it, though the fact the monitors work fine on a Mac Mini is suspicious.
What I have tried:
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Connecting the two monitors using different monitor ports (one on DisplayPort, another on HDMI, etc.), and different MacBook ports (one on HDMI, another on USB-C, etc.)
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Bumping down the resolution on the monitors to "1920x1080 (low resolution)" and 30Hz to rule out bandwidth issues.
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Connecting one, or both, monitors to CalDigit TS5 Plus dock. Neither alternate configuration yields the device recognizing both screens.
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Using BetterDisplay to import a manually-edited EDID for the screen, with a different binary EDID value, manufacturer name, etc.
I've also verified that if I plug in my Apple Studio Display as one of the monitors, then the MacBook recognizes both one of the PG32UCDM monitors and the Studio Display at the same time. The issue seems to occur only when both monitors plugged into it are the same PG32UCDM model.
When I have both monitors plugged into my MacBook, each time I disconnect the cable to whichever monitor is currently recognized, it immediately recognizes the other monitor. Plugging the cable for the disconnected monitor back in has no effect.
I'm at a loss.
Has anyone run into this issue and found a successful workaround that is not one of the approaches I've described above?
