Net debt is as meaningless as net spend. It's just total debt minus cash. Clubs get loads of cash at the end of the season, from UEFA & the PL, so cash balances are generally high at year-end, meaning net debt is lower. As they start to spend that cash, on transfers, wages and other operating expenses, it goes higher. Their total debt is almost certainly still the same.
UEFA have been very sneaky about the way they report on levels of debt under FFP. For one thing they only use net debt, taken from year end accounts. As I've explained, it's not a great measure & is generally at its lowest then. Then they report it as a percentage of revenue. So if total revenue across UEFA's clubs in 2010 was €10bn and net debt was €6bn, it'll be reported as being 60% of revenue. If in 2018 total revenue has increased to €18bn and net debt is still €6bn, it'll be reported as 33% of revenue and UEFA will claim it's fallen to nearly half the level it was in 2010. Yet the absolute level of net debt hasn't changed by a penny.
Update: Just checked their financial statements and their gross USD debt remains unchanged. However they've burned through £170m cash in the 3 months to Sep 2019, compared to a small net increase for the same period in the previous year. Not getting even into the Europa League next season could cost them another £100m cash easy.
Tick fucking tock.