Is this backlight lamp failure? (inverter already replaced)

Question

Issue:

Boots normally, but the backlight only flashes on momentarily.

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Observations:

Prior to failure the backlight had occasionally turned off completely, corrected by rebooting.

A week before failure it produced audible noise with increasing volume at lower brightness levels - by all accounts it's a common affliction of aging LCDs (likely cap failure or coil whine).

This is an all-in-one device with only HDMI in, output to an external monitor is not supported.

It's an old device, and many lamps are listed as having a 50,000 hour service life.

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Already Checked:

I've continuity-checked the inverter cable while wiggling it, and also tested most of the board's discrete components in place, as well as it's inputs with each input pin to ground (all high), and it's outputs (which are all zero, and I understand the controllers generally cease restrike attempts if the first few tries fail) - all seem fine.

I've replaced the inverter board with an identical part, with identical results.

The backlight illumination is evenly distributed and entirely white during the moment while the screen is illuminated at boot. I've inspected both lamps and they appear clean and free of discolouration. It seems unlikely to me that they'd both have failed simultaneously.

Googled and trawled Super User's backlight failure -associated questions.

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Specs:

Model: Toshiba DX1210 All-In-One
Part: PQQ09A-01U00H    
manufactured: November 2011
purchased: February 2012

OS: Win 10 64 bit