OPTIMIZE causes strange errors and further data corruption with myisam_repair_threads and too small sort buffer

-- source include/have_sequence.inc

CREATE TABLE t1 (id INT AUTO_INCREMENT PRIMARY KEY, a TEXT, b TEXT, KEY(id,a(64),b(64))) ENGINE=MyISAM;

INSERT INTO t1 (a) SELECT REPEAT('e',65173) FROM seq_1_to_50;

UPDATE t1 SET a = 'x';

SET SESSION myisam_repair_threads= 6, myisam_sort_buffer_size= 131072;

OPTIMIZE TABLE t1;

INSERT INTO t1 (a) VALUES (NULL);

# Cleanup

DROP TABLE t1;

OPTIMIZE TABLE t1;

Table	Op	Msg_type	Msg_text

test.t1	optimize	error	myisam_sort_buffer_size is too small. Current myisam_sort_buffer_size: 65536  rows: 162961  sort_length: 144

test.t1	optimize	error	Found key at page -1 that points to record outside datafile

test.t1	optimize	status	Operation failed

INSERT INTO t1 (a) VALUES (NULL);

bug.corrupt1                             [ fail ]

        Test ended at 2020-08-04 15:35:47

CURRENT_TEST: bug.corrupt1

mysqltest: At line 8: query 'INSERT INTO t1 (a) VALUES (NULL)' failed: 144: Table './test/t1' is marked as crashed and last (automatic?) repair failed

2020-08-04 15:35:46 140051098892032 [Warning] Warning: Optimize table got errno 0 on test.t1, retrying