How does a role swapped USB OTG A-device detect that it has been disconnected from the host?
As Ale..chenski referenced in their comment, section 6.3 of the On-The-Go Supplement to the USB 2.0 Specification outlines the HNP Sequence of Events:

A) A-device finishes using bus and stops all bus activity, (i.e.
suspends the bus).
B) B-device detects that bus is idle for more than TA_AIDL_BDIS min
and begins HNP by turning off pull-up on D+. This allows the bus to
discharge to the SE0 state. If the bus was operating in HS mode, the
B- device will first enter the full-speed mode and turn on its D+
pull-up resistor for at least TB_FS_BDIS min before turning off its
pull up to start the HNP sequence.
Note: After B-device enters the FS mode and turns on its pull-up
resistor; it waits to see if the data line goes high. If the data line
does not go high within TWTRSTHS (from Table 7-14 in USB 2.0
specification [USB2.0]), then the B-device shall start its HS chirp.
Otherwise, if the D+ line does go high for at least TB_FS_BDIS min,
then the B-device may start HNP.
C) The A-device detects the SE0 on the bus and recognizes this as a
request from the B-device to become Host. The A-device responds by
turning on its D+ pull-up within TA_BDIS_ACON max of first detecting
the SE0 on the bus.
D) After waiting long enough to insure that the D+ line cannot be high
due to the residual effect of the B- device pull-up, (see Section
5.1.9), the B-device sees that the D+ line is high and D- line is low, (i.e. J state). This indicates that the A-device has recognized the
HNP request from the B-device. At this point, the B-device becomes
Host and asserts bus reset to start using the bus. The B-device must
assert the bus reset (SE0) within TB_ACON_BSE0 max of the time that
the A-device turns on its pull-up.
E) When the B-device completes using the bus, it stops all bus
activity. (Optionally, the B-device may turn on its D+ pull-up when a
FS idle condition is detected on the bus.)
F) A-device detects lack of bus activity for more than TA_BIDL_ADIS
min and turns off its D+ pull-up. Alternatively, if the A-device has
no further need to communicate with the B-device, the A-device may
turn off VBUS and end the session.
G) B-device turns on its pull-up.
H) After waiting long enough to insure that the D+ line cannot be high
due to the residual effect of the A-device pull-up, (see Section
5.1.9), the A-device sees that the D+ line is high (and D- line is low) indicating that the B-device is signaling a connect and is ready
to respond as a Peripheral. At this point, the A-device becomes Host
and asserts bus reset to start using the bus.

When a B-device acting as a host suspends the bus (step E), this indicates that it is finished acting as a host. A disconnect could also be interpreted as step E, however, the occurrence of (or lack of) step G allows the A-device to differentiate between a suspend and a disconnect.
A B-device acting as a host will only suspend the bus when it is finished acting as a host.